1. Field of the Invention
The present invention relates generally to a signal mixing circuit and, more specifically, to a signal mixing circuit for use with an FM stereo receiver for producing output signals having an improved signal-to-noise ratio.
2. Description of the Prior Art
As is known, the operation of an FM radio receiver will be adversely affected and the signal-to-noise (S/N) ratio of the output signal will be objectionably reduced when the electrical field strength of the broadcast FM signal received by the receiver decreases or is weakened. Thus, in the case of an FM receiver employed in an automobile, the location of which will change over a relatively broad area, the intensity of the field strength of the broadcast FM signal received by the moble receiver will vary accordingly, and the quality of the output signal produced by the FM receiver will suffer significant degradation. This problem is exacerbated in the case of FM stereophonic signals received at an FM receiver, in which the level of the noise component contained in the left and right channel audio output signals derived from the FM receiver will increase considerably, and the quality of each of the left and right channel output signals then becomes more obviously deteriorated when the field strength of the received FM stereo signal becomes weaker.
There have been proposed numerous solutions to this S/N ratio problem, one of which is to combine the left and right channel audio output signals in order to cancel the noise components in such a fashion as to produce a blended audio output signal with the desired improved signal to noise ratio. Various problems arise in this approach in that typically the signal combination occurs at all times, thereby being unnecessary when the S/N ratio is high. Additionally, it is difficult to blend the signals continuously and gradually and to decrease the level of one of the blended channels while simultaneously increasing the lever of the other channel, which is necessary in order to produce audio signals that do not vary in level so abruptly that the resultant sounds are annoying.